Recent observations in this laboratory indicate that some cirrhotic patients, unlike healthy adults, require cyst(e)ine and/or tyrosine as essential amino acids. This is clinically significant because many enteral formulae andmost parenteral solutions in current use are lacking in one or both of these amino acids. Consequently, hospitalized cirrhotics, as surveyed here, are often deficient in cyst(e)ine and/or tyrosine. Experiments in rats have shown that an essential amino acid deficiency is a systemic, progressive, sometimes lethal disorder. Therefore, there previously unrecognized deficiencies of cyst(e)ine and tryosine must be adding to the morbidity and mortality of hospitalized cirrhotics, receiving enteral and parenteral solutions; to define the clinical circumstances that cause cyst(e)ine or tyrosine to become essential via extensive clinical, hepatic and nutritional data bases; to develop simple pharmacokinetic tests to identify those cirrhotics who have either of these dietary requirements and to develop simple tests to identify those cirrhotics who are not satisfying such requirements and have therefore developed a cyst(e)ine or tyrosine deficiency. This information will lead to new feeding solutions "custom designed" to the nutritional requirements of each cirrhotic, much as now adjust the electrolyte content of intravenous fluids to the plasma electrolyte profile of each patient. The result may then be less morbidity, less mortality, andimproved nutritional repletion of cirrhotics who require nutritional support in the hospital.